The invention concerns a method of indicating the length of an object in a measuring zone having intersecting radiation paths by determining which of the intersections have been screened-off by the object along a reference line, and a device for performing the method. Depending on the cross-sectional shape of the object, the screened-off length corresponds, as will be explained more in detail below, either to the actual length of the measured object along the reference line, in which case the invention can be used for measurement of dimension, or the screened-off length will be greater than the extent of the measured object along the reference line, and with such objects (and certainly also with the first named objects), the invention may be used for indicating the presence of the object in a measuring zone comprising the reference line, with the advantage that indication of too small objects (such as accidental dirt or refuse particles) in the measuring zone can be avoided
The objects, the dimension of which can be ascertained with sufficient accuracy according to the invention include objects with at least approximately circular cross-section having their center on the reference line. The term "approximately circular cross-section" refers to both geometrical shapes lying near a circle (such as ellipses with a minor difference between their axes or polygons with a greater number of sides), and irregular shapes similar to a circle, (such as cross-sections of logs). The invention is therefore particularly well suited for measuring dimensions in the saw mill industry, and it will be explained more in detail against this background.
In U.S. Pat. No. 3,806,253, issued on Apr. 23, 1974 to E. B. Denton a measuring device is described in which on one side of a measured object a line of emitters (light emitting diodes), and on the opposite side a line of receivers (photo-diodes), are provided at predetermined spacings. The emitters are activated one after the other and the receivers are interrogated. The cross-sectional diameter of the measured object is established with the aid of a number of receivers, which due to screening-off by the measured object, have not received light from any emitter. The best accuracy of the device, the "resolving power", is equal to one half of the spacement between the receivers, e.g. 1/4".
In the same document there is further described a device where two emitters-receiver-parts generate two radiation paths which intersect one another, and where the object is moved in the plane of its cross-section so that it first breaks one, and then the other, radiation path elsewhere then in their common point of intersection. The measured diameter is calculated as a fraction of the time, i.e. depending on the transport velocity, during which both radiation paths have been screened-off. In Swedish patent specification No. 388,272 issued on Nov. 17, 1977 to Kockums Automation AB, a measuring device for objects with trapeziod shaped cross-section, such as boards, is described where an emitter-receiver-pair also is used for generating radiation paths which intersect one another, one element in said pair possibly being doubled, e.g. one receiver associated with two emitters.
All methods and devices previously known have however one or more of the following disadvantages:
the resolving power is limited by the actual spacing of the receivers, and a greater number of receivers (needed with smaller spacings) makes the construction expensive, because receivers are much more expensive then emitters; PA0 moving parts such as rotating mirrors and the like are used which are liable to cause problems as they are dependent on temperature, and because of wear and sensitivity to vibrations also exhibit limited service life; PA0 components with very high demands on precision (such as mirrors and lenses to focus light and/or to deflect it or make it paralles) are needed which are rather expensive; PA0 components with a very reduced useful life in comparison with other parts of the system have to be used, such as lasers etc.; PA0 the measured object must be transported transversally to its length dimension, which may cause problems as it is difficult to have a log lying still on a conveyor during such a transport, and moreover, in such a case, a plurality of measuring devices in several measuring planes has to be placed along the whole length of the object along the entire length thereof.